Household air pollution (HAP) from combustion of solid fuels used for cooking and heating is one of ten leading contributors to the global burden of disease. About 90% of rural households in developing countries use solid fuel such as wood, agricultural waste or coal for cooking and heating. One epidemiologic study suggests a relationship between HAP and child neurodevelopment, but no firm conclusions could be drawn as the result was imprecise. In Sri Lanka, a lower middle-income country, more than 78% of the population use biomass fuel in the form of unprocessed wood. We have assembled a cohort of 600 mother-infant pairs in Sri Lanka to study the effects of prenatal HAP exposure on infant neurodevelopment assessed at birth and at six months of age. In this application, we propose to extend the follow-up of the same birth cohort up to 3 years of age and evaluate effects of biomass fuel smoke exposure on child's neurodevelopment through early childhood allowing us to capture the crucial periods of brain development. It will also provide an excellent opportunity to assess effect/impact of both prenatal and postnatal exposure to HAP on child neurodevelopment with minimum exposure misclassification. We will ascertain HAP exposure from questionnaires and measure PM2.5, PM1.0, PM10, CO and NO2 in 300 households. Using this information and child's time activity record, quantitative exposure profiles will be developed for the 600 children in the study. Child neurodevelopment will be assessed at 18 and 36 months of age using the Bayley Scales of Infant Development. The study will then evaluate the association between prenatal and childhood HAP exposure on child neurodevelopment. This pioneer birth cohort study addresses a global research gap, and has the potential to cause a paradigm shift in addressing HAP exposure in Sri Lanka and in other countries.